Field of the Invention
The present invention relates to a method for manufacturing a rectangular parallelepiped-shaped single crystal containing sodium niobate of a perovskite structure as the main component. The present invention also relates to a method for manufacturing a ceramics using a rectangular parallelepiped-shaped single crystal containing sodium niobate as the main component, the ceramics, a piezoelectric element, a piezoelectric device, and an electronic device.
Description of the Related Art
As template grains for use in manufacturing of an oriented piezoelectric ceramics containing sodium niobate of a perovskite structure as the main component, sodium niobate plate-like particles and sodium niobate rectangular parallelepiped-shaped single crystal are known.
As former template grains of sodium niobate, plate-like particles have been mainly used and have been manufactured by a two-step molten salt synthesis. “Ferroelectrics” (M. Cao et al., 2010, 404th volume, 39 page to 44 page) discloses a method for manufacturing sodium niobate plate-like particles by the two-step molten salt synthesis. Moreover, “Chemistry of Materials” (S. F. Poterala et al., 2010, 22th volume, 2061 page to 2068 page) discloses the mechanism that the plate-like particles manufactured by the two-step molten salt synthesis become a polycrystal.
Specifically, plate-like particles of a bismuth layer-structured compound produced first in the first stage of a synthesis process are rectangular parallelepiped crystals. However, a topochemical reaction involving the structural change from the bismuth layer structure to a perovskite structure, which occurs in the second stage of the synthesis process, progresses towards the inside from the front surface of the plate-like particles of the bismuth layer-structured compound which is an intermediate. Therefore, a liquid-phase boundary layer containing a large amount of bismuth is formed on the interface of a reacted portion of the plate-like particle surface and an unreacted portion inside the plate-like particles, and epitaxy is lost by the boundary layer. It is disclosed that, as a result, the plate-like particles do not become rectangular parallelepiped-shaped single crystals but become polycrystals.
In recent years, as the template grains of sodium niobate, those having a shape of a rectangular parallelepiped-shaped single crystal have drawn attention. “Japanese Journal of Applied Physics” (K. Ishii et al., 2013, 52th volume, 09KD04-1 page to 09KD04-6 page) discloses a method for manufacturing a rectangular parallelepiped-shaped single crystal of sodium niobate manufactured by a single-step molten salt synthesis. Specifically, it is disclosed that a platinum crucible filled with mixed powder of sodium carbonate, sodium chloride, niobium oxide, and bismuth oxide weighed into a predetermined molar ratio is held at 1225° C. for 12 hours, and then the obtained product is washed two or more times in warm water to remove the sodium chloride. Then, the by-product is removed through a 250 mesh sieve, whereby a rectangular parallelepiped-shaped single crystal of sodium niobate of a perovskite structure is obtained. According to the single-step molten salt synthesis described in “Japanese Journal of Applied Physics” (K. Ishii et al., 2013, 52th volume, 09KD04-1 page to 09KD04-6 page), in the formation of a rectangular parallelepiped-shaped single crystal of sodium niobate, the sodium niobate plate-like particles are produced via Bi2.5Na3.5Nb5O18 (m=5) which is a bismuth layer-structured compound as an intermediate in the same manner as in the former two-step molten salt synthesis. After the particles of the bismuth layer-structured compound which is an intermediate are formed, sodium niobate is generated by the topochemical reaction, and then a rectangular parallelepiped-shaped single crystal is obtained. The heat treatment process in the single-step molten salt synthesis is performed once and the acid washing is unnecessary. Furthermore, the grown rectangular parallelepiped-shaped single crystal has a smooth front surface.
The plate-like particles obtained by the two-step molten salt synthesis require performing heat treatment twice and an acid washing process for removing bismuth oxide. Furthermore, since the grown plate-like particles are polycrystals and the surface unevenness is large, the plate-like particles are not suitable as template grains for producing the oriented ceramics, and thus the plate-like particles have had various problems also from the viewpoint of industrial production and the viewpoint of the substance characteristics.
When the present inventors performed a supplementary examination referring to “Japanese Journal of Applied Physics” (K. Ishii et al., 2013, 52th volume, 09KD04-1 page to 09KD04-6 page) and then measured the composition of the obtained rectangular parallelepiped-shaped single crystal of sodium niobate. Then, the molar ratio of the sodium per mole of the niobium was less than 0.8 and greatly deviated from the stoichiometric ratio. This caused a problem that the composition of an oriented ceramics containing sodium niobate produced using the rectangular parallelepiped-shaped single crystals of sodium niobate produced by a single-step molten salt synthesis used heretofore as the template grain deviated from the stoichiometric ratio.
The present invention has been made in order to solve such problems and provides a method for manufacturing a rectangular parallelepiped-shaped single crystal containing sodium niobate as the main component having a perovskite structure and having a sodium deficiency amount to niobium of 20% or less, a method for manufacturing an oriented ceramics using the rectangular parallelepiped-shaped single crystal, and an oriented ceramics containing the same.
The present invention also provides a piezoelectric element, a piezoelectric device, and an electronic device containing the oriented ceramics.